The apolipoprotein composition of plasma lipoproteins is governed not only by the composition of the nascent lipid-apolipoprotein complex, but also depends critically upon the concentration and composition of other components of plasma, including other plasma lipoproteins. The distribution of the majority of apolipoproteins among plasma lipoproteins is governed by the laws of mass action and can in many cases be treated in terms of equilibrium constants. The redistribution of apolipoproteins with metabolism can be viewed in terms of perturbations on these equilibrium constants. This work is directed towards a greater understanding of the molecular forces involved in the quaternary organization of plasma lipoproteins. In order to quantitate these interactions we have investigated the solution properties of apolipoproteins. These studies have been extended to include an analysis of the molecular properties of modified apolipoproteins and also quantitation of "mixed" interactions between apolipoproteins in aqueous solution. These "mixed" interactions are of high affinity and result in formation of specific mixed oligomers. The oligomers (homogeneous and heterogeneous) of apolipoproteins in plasma are undoubtedly of major importance in the quaternary organization of plasma lipoproteins.